Pierre Kleiber
Southwest Fisheries Science Center
National Marine Fisheries Service
P.O. Box 271
La Jolla, California 92038-0271 USA
ABSTRACT
The principal categories of fisheries interactions are economic competition, stock-mediated interaction, and gear competition. While all three types of interaction may be important, this paper focusses on classifying stock-mediated interactions. Within stock-mediated interactions, there can be direct competition between fisheries, either concurrent or consecutive, and secondary effects on recruitment or trophic relationships. In addition, stock-mediated interactions may depend on movement of the fish. The analysis and assessment of any interaction must take into account the particularities of the situation.
1. TYPES OF INTERACTIONS
As a contribution to the introductory agenda item of this Second FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, I was asked to offer a typology of fishery interaction. This is essentially the same task I had for the first consultation (Kleiber, 1994). Little has changed in the intervening time to suggest a revised typology, but rather than simply copying the text of my previous paper, my plan is to concentrate on a pictorial presentation that was lacking in that paper.
Figure 1 shows a schematic typology of mechanisms by which the activity of one fishery could affect the performance of another fishery. That promulgation of cause and effect from one fishery to another is what we call fishery interaction. The various categories were described at the previous consultation (Kleiber, 1994; Hampton, 1994). Principal categories are: 1) economic competition, where the market activity of one fishery affects the market conditions (e.g., price) of another fishery; 2) stock-mediated interaction, where one fishery has some effect on the abundance or availability of the target fish population of another fishery; and 3) gear competition, where deployment of gear or other operations of one fishery interferes with the operations of another fishery. As before, I have concentrated on stock-mediated interaction. Within that category we have direct competition between fisheries aiming to harvest the same individual animals, and we have secondary effects in which one fishery impacts the resource of another fishery by some means other than removal of individuals that the other fishery is otherwise destined to catch. Direct competition can either be concurrent within the life-cycle of the fish, with fisheries harvesting the same sizes or ages, or it can be consecutive, with different fisheries harvesting different sizes, ages, or life stages. A possibly important secondary effect could occur if one fishery depresses the spawning stock which in turn would affect subsequent recruitment to another fishery. Other secondary effects could be mediated by trophic relationships in which the species harvested by one fishery is either food for, or a predator of, the resource stock of another fishery. An important cross-categorization within stock-mediated interaction is based on whether competing fisheries are spatially coincident or remote from each other, that is, whether the interaction effect depends on the movement of fish.
Figure 1. Schematic typology of fishery interaction mechanisms.
Figure 2 represents processes by which various types of stock-mediated interaction can occur by means of a generalized tuna life-cycle with fisheries superimposed on it at various stages. This figure illustrates the fact that binary categories in the typology can be extremes of a continuum. Thus, the distributions of size in the catch of two fisheries can overlap completely, partially, or not at all, and the areas in which fisheries operate can overlap completely or partially, or can be separated by short or long distances.
The schematic structure in Figure 1 relates to strategies for assessing interaction and in particular to the design and complexity of assessment models. With strictly concurrent interaction, knowledge of the exploitation rate in each fishery would be sufficient information for assessments. With consecutive interaction it would be necessary to add size, age, or life stage structure to the model. Recruitment interaction would require a stock-recruitment relationship, and trophic effects would require a multi-species model. Whatever the level of population dynamic complexity, going from strictly coincident interaction to remote interaction requires the addition of diffusive movement, advective movement or both to the model.
Figure 2. Generalized tuna life-cycle illustrating mechanisms of stock-mediated fishery interaction. The circles shaded with a pattern of boats are fishing fleets impinging on various stages in the tuna life cycle which is represented by circular or oval areas (except for one circle on the bottom which represents tuna prey or tuna predators). The arrows indicate some combination of growth, development, and geographic or trophic movement of fish. The rectangles contain histograms of size or age in the catch.
My emphasis on stock-mediated interaction reflects my inclination and training in population dynamics, but it does not imply that other mechanisms are of lesser importance. Gear interference might be important in some situations, and it is quite possible that economic competition between tuna fisheries is of greater consequence than stock-mediated interaction. For some tuna fisheries it can be said that the market is more of a limiting resource than the productivity of the fish population. A fisheries economist would no doubt be able to subdivide that category in useful ways that would reflect strategies for assessing economic fishery interaction. As with most typologies, there are some things that defy categorization. For example, net marking of escapees from gill nets reduces their marketability when they are later caught by other gear. That interaction could be said to be a hybrid of economic, stock, and gear interaction. The lesson is that analysis and assessment of interaction in any particular situation must take into account the particularities of that situation, generalized typologies notwithstanding.
2. REFERENCES CITED
Hampton, J. 1994. A review of tuna fishery-interaction issues in the western and central Pacific Ocean. In: Shomura, R.S., J. Majkowski and S. Langi (eds.). Interactions of Pacific tuna fisheries. Proceedings of the First FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 3-11 December 1991, Noumea, New Caledonia. Vol. 1: Summary report and papers on interaction. FAO Fish. Tech. Pap. (336/1): 138-157.
Kleiber, P. 1994. Types of tuna fishery interaction in the Pacific Ocean and methods of assessing interaction. In: Shomura, R.S., J. Majkowski and S. Langi (eds.). Interactions of Pacific tuna fisheries. Proceedings of the First FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 3-11 December 1991, Noumea, New Caledonia. Vol. 1: Summary report and papers on interaction. FAO Fish. Tech. Pap. (336/1): 61-73.
